Internal-combustion engines have recently been improved in many respects so as to cope with the growing demands for saved fuel oil, sophisticated engine performance, reduced body weight, lessened exhaust gas and the like. In particular, a piston ring adapted for expansibly slidable movement inside the cylinder of such an internal-combustion engine has been required to provide reduced thickness in attaining weight lightness and to offer improved fatigue resistance, wear resistance, scuffing resistance and like properties in ensuring high-speed rotation. To meet these physical requirements, a piston ring made of steel is taking the place of a conventional counterpart made of cast iron because the former is rather easy to reduce its weight, great in mechanical strength and highly resistant to fatigue.
As regards the above-stated, steel piston ring, its top ring (pressure ring) in common use is based on a Si-Cr steel or a martensitic stainless steel having 11 to 17% Cr and resulting from chromium plating or nitriding treatment. A second ring of that piston ring is at present dominantly of cast iron which, however, is being replaced with steel in response to the physical qualities desired above. The cast iron-made second ring itself is subjected to chromium plating or nitriding treatment prior to being practically used.
Oil rings, which are so assembled as to scrape a fuel oil during sliding-contact with the cylinder, are generally classified into two different types, i.e. three-piece and two-piece types. The three-piece type of oil ring has two side rails held in directly slidable relation to the cylinder and formed mainly from such a material as is composed with a wear resistance property taken as particularly important. This material is made rich in Cr content (17 Cr) and subjected to nitriding treatment. The two-piece oil ring is structured with a shaped-section wire material having a not simple sectional figure.
In the production of the foregoing piston rings, several different materials are employed according to the constitutive portions which depend on the structural types of internal-combustion engines to be designed and on the physical properties to be achieved. Namely, in the case where piston rings are mass-produced, their slidably movable portions are formed from a 0.8-0.9 C-17 Cr steel which may if appropriate contain Mo and V. This is due to high resistances to wear, scuffing and corrosion called for as important qualities. For top rings generally for use in gasoline engines, 0.7 C-12 Cr steel and the like are used in which Mo and V may optionally be incorporated.
As prior publications relating to the piston ring technology, JP-B2-57-8302 is cited to disclose a pressure ring for use as a piston ring, JP-B2-58-4654 to disclose a steel material for formation of a piston ring and JP-B2-2-4829 to disclose a side rail for an oil ring.
In producing the steel piston ring, a flat wire material or a shaped-section wire material is heated at from 900 to 1,100.degree. C. and thereafter annealed with quenching, followed by tempering of the thus treated material at rather high a temperature, so that the resulting material is adjusted in its hardness after heat treatment to a somewhat low level of about 38 to 52 HRC. Such a smaller hardness is intended to gain good bending working at a subsequent process step of shaping that material into a piston ring. To be more specific, a greater hardness is preferred when the desired qualities of the piston ring product, such as wear resistance, scuffing resistance and the like, are taken as primary. In this instance, however, the starting material causes objectionable breakage in the course of bending working. Common practice resides in holding the hardness after heat treatment at a low level even with a slight sacrifice to the desired qualities of the ultimate piston ring. After being heated-treated with continuous bending working, the flat wire or shaped-section wire material is severed to individual rings that are subsequently subjected to thermal correction and to surface treatment (as by plating or nitriding), whereby a piston ring product is provided.
On the other hand, the martensitic steel poses the problem that, because it can be highly work-hardened, a high working rate cannot be attained in finishing with a flat wire or a shaped-section (or modified cross section) wire. This entails many cycles of annealing at a stage of drawing working or rolling working, thus resulting in increased cost.